The present invention relates to a gas scrubber, as it is known, which comprises at least one drop separator level through which a gas stream flows vertically, drop separators being installed in an, in particular, roof-shaped set-up. The invention is employed particularly preferably in the sector of flue-gas desulphurization.
The combustion of coal gives rise, inter alia, to sulphur dioxide gas which is a substantial cause of the death of forests. There are various methods for extracting the harmful sulphur dioxide from the flue gas. The wet method, as it is known, is used most often. In this, the unpurified flue gas is sprayed in a washing tower, also called an absorber tower or gas scrubber, with a mixture of water and limestone, what is known as a washing suspension, with the result that the sulphur dioxide is largely absorbed due to chemical reactions. It is thus possible to achieve a degree of desulphurization of more than 90%. In this case, the gaseous sulphur dioxide first dissolves in the washing liquid. Subsequently, due to the reaction of sulphur dioxide and limestone, calcium sulphite and carbon dioxide are obtained. The washing suspension laden with calcium sulphite collects in the lower part of the washing tower, in the absorber sump. By air being injected (oxidation), the liquid is enriched with oxygen, and a gypsum suspension occurs. After the water has been extracted, gypsum with a residual moisture of up to 10% is obtained in pourable form and is available as a useful product for delivery to the building materials industry.
The drop separators, as a rule, are installed downstream of the gas scrubbing in the gas flow direction and cover the entire cross section of the usually round gas scrubber tower. The drop separator is in this case formed by curved lamellae which lie parallel to and at a defined distance from one another and on which the drops contained in the gas flow are precipitated. The precipitated drops form a liquid film which, obeying the law of gravity, flows off downwards or falls in large drops downwards counter to the gas stream.
Since flue gas is heavily laden with fly ash and gypsum is formed during the further desulphurization process, there is always the risk that these solid particles are deposited on the drop separator and may even block this. Consequently, below the respective separator layer and often also above it (downstream of the drop separator in the gas flow direction), scavenging devices are installed, which periodically wash the drop separator lamellae and eliminate possible deposits. This scavenging device consists, inter alia, of pipes with nozzles inserted in them.
Drop separators set up in a roof-shaped manner, that is to say configurations with a V-shaped arrangement of inclined lamellae, have proved (particularly in the case of vertical gas stream) to be advantageous (for example, as compared with flat drop separators lying horizontally) both as regards cleaning off and keeping clean and in terms of a reliable separation performance. The drop separator lamellae of streamlined shape deflect the gas stream laden with liquid. The drops cannot perform this deflection because of their inertia, but, instead, impinge onto the drop separator lamellae (rebound-surface separator). In this case, a liquid film is obtained, which then for a large part flows off downwards along a suitable profile. In order to adapt the performance to the said object, the drop separators are offered with special shapes and properties. Consequently, the reliable removal of the liquid is ensured, while at the same time there is a high separation performance. Conventional forms of construction of these drop separators with inclined drop separator lamellae are known, for example, from DE 195 01 282 or DE 195 21 178. The roof-shaped drop separator is meanwhile being used by many power stations on account of these advantages.
A critical advantage of the roof-shaped drop separator is the reliable separation performance at high vertical gas velocities of more than 5 m/s, up to inflow gas velocities of 6.5 to 7.5 m/s, depending on the configuration. Conventional flat (horizontally lying) drop separators have their performance limit at 5.2 to 5.5 m/s (vertical inflow gas stream). The higher performance limit of roof-shaped drop separators is advantageous particularly in the operation of large plants for large-scale power stations. On account of the non-uniform gas flow in the gas scrubber, operational conditions and the structural configuration give rise in these gas scrubbers, which have, for example, a diameter of 12 m to 17 m and are operated under full load at a basic velocity of 3.5 m/s to 3.8 m/s, to local velocity peaks of 5 m/s to 6 m/s and, in individual instances, even more than this. Such velocity peaks lead, in conventional flat drop separators, to local failure and therefore considerable supercritical drop overflow. The performance of the overall drop separator is thereby reduced considerably, and contamination of the following plants in the flue-gas duct occurs.
As a result of these developments, the performance requirements have once more increased markedly. More modern plants are operated at higher basic velocities, for example of between 4.0 m/s and 4.5 m/s. Furthermore, markedly higher fluctuations of the basic velocity may occur locally. Even local velocity peaks of up to 10 m/s have been observed in individual instances.
The evaluation of these velocities must take into account the fact that the inflow velocity of the drop separator is, in turn, 15% to 25% higher than the basic velocity in the plant. In the region of the drop separator, the open gas-through flow cross-sectional area narrows due to carrying beams (on which the drop separator lies), because of the structural configuration of the drop separators and on account of the blinding of individual regions. This leads to a further rise in the basic velocity and to an even higher inflow velocity for the drop separator. An inflow velocity means that velocity of the gas stream which prevails or acts in the region of the drop separator. Basic velocities of 4.0 m/s to 4.5 m/s become an inflow velocity of 5.0 to 5.5 m/s. Correspondingly, velocity peaks of 6-8 m/s become inflow velocity peaks of 7.5 to 10 m/s, in individual instances up to 12 m/s. These velocities overtax even the currently conventional roof-shaped drop separators.
At the same time, the frequent problems with contaminations in the heat exchanger following drop separators have made power stations more sensitive with regard to the performance problems of the drop separator. The requirements as to pressure loss, the duration of an operating cycle and the characteristic value “residual content of drops in the flue gas” downstream of the drop separator have markedly intensified. Where, 10 years ago, residual contents of 100 to 150 mg/m3 were still required, nowadays 30 to 50 mg/m3 are mostly required as a guaranteed value for the residual content. The conventional roof-shaped drop separator meanwhile comes up against its performance limits under these conditions.
Both trends, to be precise the higher basic velocity with the higher velocity peaks, on the one hand, and the intensified requirements as to separation performance, on the other hand, point to the need to develop further the gas scrubbers and drop separators known hitherto.
In the known gas scrubbers, the individual drop separator levels are implemented in that a carrying structure comprising carrying beams and/or side rings is provided, on which a plurality of drop separator modules are positioned. As a rule, the modules are exchangeable and have a square or rectangular cross section. These modules, in so far as they have fitted into the spaces between the carrying beams or between the carrying beams and a side ring of the gas scrubber, have been installed, for example, in order to keep the costs of such a gas scrubber low. The still free flow cross sections have subsequently been closed (blinded) by means of plates or the like, so that the overall gas flow has been forced through the drop separators. In gas scrubbers of this type, however, insufficient purification or final humidification of the gas stream has been noted.